Process for the continuous production of a preform mat, and a preform and its use

ABSTRACT

From about 50 to about 90% by weight of fibers, in particular wood fibers, and/or pellets made from cellulose or from wood, are mixed with from about 10 to about 50% by weight of a hot-curing resin, each of the percentages by weight being based on the total weight of the mixture. Instead of a single resin it is also possible to use a mixture made from two or more hot-curing resins. The mixture made from fibers and/or pellets and resin is kneaded and homogenized and then passed on to an extruder which has various heating zones in which the mixture is heated. Care has to be taken here that the maximum temperature, arising at the extruder exit, does not exceed about 120° C., since above this temperature the resin(s) begin(s) to cure. The mixture, not entirely cured, is extruded as a homogeneous composition and introduced into a calender line in which pressure is exerted on the composition to an extent which gives a preform mat, the shape of which is stabilized, and which is then removed for further processing or storage.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.09/788,751 filed Feb. 20, 2001 now U.S. Pat. No. 6,582,640, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for the continuous production of amat which is a preform for sheets, made from a mixture composed offibers and/or pellets made from cellulose or wood and at least onehot-curing resin, and also to a preform and its use.

2. Description of the Related Art

U.S. Pat. No. 5,082,605 discloses a process for the production of acomposite material which comprises a discontinuous phase of cellulosefibers, bonded and encapsulated within a continuous phase of a polymericcomponent, the polymeric component being composed mainly ofpolyethylene. The cellulose fibers have a ratio of length to diameter offrom 2 to 12 and are mixed with the polymeric component, the temperatureof the mixture being increased as far as the point of encapsulation. Theencapsulated material is held within the encapsulation range while theparticle size is reduced. The material is then extruded while itstemperature is controlled within the encapsulation range. The fibers areessentially oriented in the direction of flow until the material reachesa heated die. Cellulose fibers and the polymeric material are mixed in aratio of between about 40% by weight of polymer and 60% by weight offibers and 60% by weight of polymer and 40% by weight of fibers, untilthe surface temperature of the mixture is between 143 and 177° C., oreven higher. The surface temperature here depends on the polymericmaterial used. The mixture is then divided into portions whose maximumdimension is about 3.8 cm. These portions are then extruded, thetemperature being held within the encapsulation temperature range.During the extrusion, the encapsulated fibers are essentially orientedin the direction of flow. The polymer is mainly a low-densitypolyethylene, but there may be a relatively small content ofhigh-density polyethylene. There may also be a relatively small contentof polypropylene in the polymer. Examples of products manufactured fromthe composite material are roofing shingles, floor tiles, panels, boardsor frames for doors or for windows, and also items for the household orfor gardens.

U.S. Pat. No. 4,228,116 describes formable sheets produced by continuousplastification and extrusion of an at least partially aggregated mixturemade from 40 to 60% by weight of a thermoplastic resin, preferablypolypropylene, and from 40 to 60% by weight of an organic, in particularcellulosic, filler material, such as wood particles. Before curing, theextruded material is rolled out to give a strip. This extruded,rolled-out strip is cut into sheets and these are used to manufacture,for example molded items, such as dashboards, instrument panels or othercomponents for vehicles, or furniture or parts for furniture, or thelike, by working the sheets at elevated temperature via pressing orother methods of forming, as are known in the prior art for theprocessing of thermoplastic materials. As an alternative, the sheets maybe used for purposes for which sheets of timber or plywood are normallyused.

WO 91/19754 discloses a mat impregnated with plastic and composed of asolvent-free plastic based on an epoxy resin and on a phenol novolakhardener, with an accelerator if desired. The proportion of volatilesubstances is below 0.5% by weight. To produce this mat, a solvent-freeepoxy resin-phenol novolak mixture is heated to a temperature of from 60to 140° C., at which the resin mixture has a viscosity of from 5 to 50Pa.s. The preheated epoxy resin-phenol novolak mixture continuouslypermeates a web of fiber mat in a calender. The mat produced by thisprocess has good storage stability and is suitable for producinghot-pressed, fully cured plastic parts with high resistance to chemicalsand heat. An example of the fiber mat is a glass fiber mat.

It is a feature of the invention to provide a process which can extrudepreforms with up to about 90% by weight of organic fiber content byconventional methods, without decomposition of the mixture. The featureextends to providing storage-stable preforms which can be used to moldfully cured sheets or sheet sections which have a substantially smoothor, where appropriate, a slightly structured surface.

This feature is achieved by means of a process which comprises:

(a) mixing from about 50 to about 90% by weight of fibers and/or pelletswith from about 10 to about 50% by weight of hot-curing resin, whereeach of the percentages by weight is based on the total weight of themixture,

(b) kneading and homogenizing the mixture made from fibers and/orpellets which break down to give fibers, and from at least onehot-curing resin, and passing the mixture onward to an extruder,

(c) heating the mixture in the extruder to an extruder exit temperatureat which the resin begins to cure,

(d) extruding the mixture, which has not entirely cured, as ahomogeneous composition and distributing the homogeneous compositionwithin a calender line,

(e) exerting pressure onto the homogeneous composition in the calenderline, to an extent which gives a mat which is a preform, and

(f) removing the preform and stabilizing its desired shape.

Another procedure which achieves the feature is that of:

(a′) mixing and kneading from about 50 to about 90% by weight of fibersand/or pellets which break down to give fibers, with from about 10 toabout 50% by weight of hot-curing resin in an extruder,

(b′) varying extents of heating in the extruder of the mixture made fromfibers and from at least one hot-curing resin,

(c) heating the mixture in the extruder to an extruder exit temperatureat which the resin begins to cure,

(d) extruding the mixture, which has not entirely cured, as ahomogeneous composition and distributing the homogeneous compositionwithin a calender line,

(e) exerting pressure onto the homogeneous composition in the calenderline, to an extent which gives a mat which is a preform, and

(f) removing the preform and stabilizing its desired shape.

The fibers used in the process are wood fibers whose moisture level isin the range from about 2 to about 50% by weight, in particular fromabout 10 to about 20% by weight. It is useful to knead the mixture in amixer and then pass the same onward to an extruder from which themixture is extruded without any back-pressure being exerted on theextruder screw. The hot-curing resins used in the process comprisethermosetting polymer compositions and/or resins with different extentsof condensation, or are different types, or are mixtures of two or moreof these resins. It is advantageous for the introduction of the resinsto the mixer to be separate from that of the fibers and/or pellets. Theextruder comprises zones with different temperatures, the temperaturebeing controlled so that the temperature of the mixture is not more thanabout 120° C. at the exit from the extruder. The resin or thermosettingpolymer composition preferably starts to cure or polymerize at thistemperature which temperature however is preferably not high enough forfull and entire polymerization or curing of the resin or of the mixturemade from resin with fibers. In one embodiment of the process, thehot-curing resins have been selected from the group consisting ofthermosetting polymers, phenolic resins, melamine resins, and epoxyresins.

The preform for hot-pressed, fully cured sheet sections or sheets madefrom a composite material made from fibers and from at least onehot-curing resin is composed of from about 50 to about 90% by weight offibers and from about 10 to about 50% by weight of at least onehot-curing resin, the percentages by weight being based on the totalweight of the composite material, where the resin of the preform has notentirely cured.

The preform is used as starting material for hot-pressed, fully curedsheet sections or sheets, by inserting one or more plies into a mold andfully curing these by hot-press molding.

SUMMARY OF THE INVENTION

The invention provides a process for the continuous production of a matwhich is a preform for sheets, made from a mixture composed of fibersand/or pellets made from cellulose or from wood and at least onehot-curing resin, comprising the steps of

(a) mixing from about 50 to about 90% by weight of fibers and/or pelletswith from about 10 to about 50% by weight of hot-curing resin, whereeach of the percentages by weight is based on the total weight of themixture,

(b) kneading and homogenizing the mixture made from fibers and/orpellets which break down to give fibers, and from at least onehot-curing resin, and passing the mixture onward to an extruder,

(c) heating the mixture in the extruder to an extruder exit temperatureat which the resin begins to cure,

(d) extruding the mixture, which has not entirely cured, as ahomogeneous composition and distributing the homogeneous compositionwithin a calender line,

(e) exerting pressure onto the homogeneous composition in the calenderline, to an extent which gives a mat which is a preform, and

(f) removing the preform and stabilizing its desired shape.

The invention further provides a process for the continuous productionof a mat which is a preform for sheets, made from a mixture composed offibers and/or pellets made from cellulose or from wood and at least onehot-curing resin, comprising the steps of

(a′) mixing and kneading from about 50 to about 90% by weight of fibersand/or pellets which break down to give fibers, with from about 10 toabout 50% by weight of hot-curing resin in an extruder,

(b′) varying extents of heating in the extruder of the mixture made fromfibers and from at least one hot-curing resin,

(c) heating the mixture in the extruder to an extruder exit temperatureat which the resin begins to cure,

(d) extruding the mixture, which has not entirely cured, as ahomogeneous composition and distributing the homogeneous compositionwithin a calender line,

(e) exerting pressure onto the homogeneous composition in the calenderline, to an extent which gives a mat which is a preform, and

(f) removing the preform and stabilizing its desired shape.

The invention still further provides a preform for hot-pressed, fullycured sheet sections or sheets made from a mixture made from fibers andfrom at least one hot-curing resin, wherein the mixture is composed offrom about 50 to about 90% by weight of fibers and from about 10 toabout 50% by weight of at least one hot-curing resin, where thepercentages by weight are based on the total weight of the mixture, andwherein the resin of the preform has not entirely cured.

The invention still further provides a process for the continuousproduction of a mat comprising:

(a) forming a mixture of from about 50 to about 90% cellulose or woodfibers and/or pellets by total weight of the mixture and from about 10to about 50% of at least one thermosetting polymer composition by totalweight of the mixture;

(b) kneading the mixture until substantially uniform;

(c) heating the mixture in an extruder to a temperature at which thethermosetting polymer composition of the mixture begins to at leastpartially polymerize, thereby forming a partially polymerized mixture;

(d) extruding the partially polymerized mixture as a homogeneouscomposition, and distributing the homogeneous composition within acalender line;

(e) exerting pressure on the homogeneous composition in the calenderline, to thereby form mat which is a perform; and

(f) removing the perform from the calender line.

The invention still further provides a process for the continuousproduction of a mat comprising:

(a) forming and kneading a mixture of from about 50 to about 90%cellulose or wood fibers and/or pellets by total weight of the mixtureand from about 10 to about 50% of at least one thermosetting polymercomposition by total weight of the mixture in an extruder until themixture is substantially uniform;

(b) heating the mixture to various temperatures in the extruder tothereby modify and control the density of the mixture;

(c) heating the mixture in the extruder to a temperature at which thethermosetting polymer composition of the mixture begins to at leastpartially polymerize, thereby forming a partially polymerized mixture;

(d) extruding the partially polymerized mixture as a homogeneouscomposition and distributing the homogeneous composition within acalender line;

(e) exerting pressure on the homogeneous composition in the calenderline to thereby form a mat which is a perform; and

(f) removing the perform from the calender line.

The invention still further provides a preform for sheets formed from amixture comprising cellulose or wood fibers and/or pellets and from atleast one thermosetting polymer composition, wherein the mixturecomprises from about 50 to about 90% cellulose or wood fibers and/orpellets by total weight of the mixture and from about 10 to about 50% ofat least one thermosetting polymer composition by total weight of themixture, and wherein the thermosetting polymer composition of thepreform has at least partially polymerized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram for the individual steps in a first methodof executing the process of the invention.

FIG. 2 shows a flow diagram for the individual steps in a second methodof executing the process of the invention.

FIG. 3 shows a diagram of equipment for carrying out the second methodof executing the process of the invention, as shown in FIG. 2.

FIG. 4 shows other equipment for carrying out the process, as in thediagram of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a process which can extrude preforms with up toabout 90% by weight of organic fiber content by conventional methods,without decomposition of the mixture. This extends to providingstorage-stable preforms which can be used to mold fully cured sheets orsheet sections which have a substantially smooth or, where appropriate,a slightly structured surface.

The preform is preferably from a composite material made from fibers andfrom at least one hot-curing resin is composed of from about 50 to about90% by weight of fibers and from about 10 to about 50% by weight of atleast one hot-curing resin, the percentages by weight being based on thetotal weight of the composite material, where the resin of the preformhas not entirely cured.

The preform is used as starting material for hot-pressed, fully curedsheet sections or sheets, by inserting one or more plies into a mold andfully curing these by hot-press molding.

One of the important constituents used in the process of the inventionis wood particles, such as wood fibers, pellets, wood chips and/or woodflour. Lower lengths and diameters of the wood particles give smoothersurfaces of the resultant preforms for sheets or sheet sections andallow higher densities of these preforms to be achieved. The woodparticles used in the processes generally have a diameter or a width ofless than/equal to about 3 mm and a length of from less than/equal toabout 6 mm to less than/equal to about 60 mm, and the length/width ratiois therefore in the range from about 2 to about 20. If the wood chipshave a length to width ratio of up to about 20, the resultant preformshave very smooth surfaces. If the width of the wood particles becomessmaller at constant length, or if the width remains the same while thelength of the wood particles increases, the ratio of length to width canvary in the range from about 21 to about 40, and the surface of theresultant preforms is less smooth than the surface of the preforms whichcomprise thicker or, respectively, shorter wood particles. For certaintypes of sheets in which a rough surface structure is desirable it ispossible to use preforms manufactured from elongated wood particles orwood fibers.

Fibers are moreover understood to be cellulose fibers and/or woodfibers, and the pellets also embrace both wood pellets and cellulosepellets. Pellets are used mainly when the bulk density of the mixture isto be higher than can be achieved using loose fibers.

The fibers are generally dried in advance so that their moisture contentis from about 2 to about 50% by weight, in particular from about 10 toabout 20% by weight. Like the resins, the fibers may also, of course, bedried prior to extrusion to a moisture content below about 10% byweight, and in particular from about 5 to about 2% by weight, ifinexpensive energy is available for the drying procedure. Excessivemoisture in the fibers adversely affects their bonding to the secondimportant constituent in the process, namely a hot-curing resin or amixture made from at least two hot-curing resins. Excessive moistureincreases the processing time and the energy cost required to reduce themoisture level of the fibers. If the initial moisture level is notexcessively high, up to about 20% by weight, no predrying of the fibersis required, since a high proportion of water evaporates due to the heatgenerated in the extruder and the large surface areas of the fibers,which have some degree of separation from one another when they leavethe extruder, and therefore the moisture content of the preforms isbelow about 10% by weight, and in most cases indeed below about 7% byweight. This is a requirement for avoiding the occurrence of blisters orcraters in the preform.

The hot-curing resin(s) used in the process comprise at least onethermosetting polymer composition and/or resin with different extents ofcondensation, or are different types, or are mixtures of two or more ofthese resins. In one embodiment of the process, the hot-curing resinshave been selected from the group consisting of thermosetting polymers,phenolic resins, melamine resins, epoxy resins, and combinationsthereof. Such hot-curing resins preferably have a viscosity of at leastabout 0.05 Pa.s. The range of viscosity of the resins can extend fromabout 0.05 to about 100 Pa.s. Where possible, the resins are free fromwater and/or solvent. If the resins comprise water or a solvent, thisconstituent of the resins is removed by appropriate known methods duringthe extrusion or after the calendering of the composite materialcomposed of one or more hot-curing resins and of the fibers. Appropriatethorough predrying of the fibers and of the resins prior to extrusioncan substantially reduce the requirement for disposal of waste water,since very little waste water then arises.

If required, additives from the group consisting of mineral particles,radiation stabilizers, catalysts, and color pigments are added to themixture made from fibers and/or pellets and from hot-curing resins.

In one preferred version of the invention, from about 50 to about 90% byweight of fibers and/or pellets are mixed with from about 10 to about50% by weight of one or more hot-curing resins, each of the percentagesby weight being based on the total weight of the mixture. The relativepercentages by weight of the fibers and/or pellets and of the resin(s)are variable and depend on various parameters, such as the type andparticle size, and also moisture level in the fibers and/or pellets, andon the viscosity of the resin(s). By altering these percentages byweight, the physical properties of the mixture from which the preformsare produced can be adjusted as desired.

As shown in the diagram of FIG. 1, hot-curing resin 2 and fibers and/orpellets 3 are introduced separately to a mixer 1. Two types of fibersand/or pellets may be fed here. Instead of a resin, it is also possibleto supply the mixer 1 with a mixture made from two or more hot-curingresins. It is also possible to feed more than two types of fibers and/orpellets, differing from one another in density and length to diameterratio. The use of pellets made from cellulose or from wood instead ofloose fibers can substantially increase the bulk density achieved, by upto about 100%. The use of pellets instead of fibers simplifies thefeeding procedure. Depending on the bulk density required, use is madeonly of fibers, only of pellets, or else of combinations of fibers andpellets.

The mixer 1 comprises kneading equipment, and the fibers and/or pellets3 are homogeneously incorporated into the hot-curing resin 2. It isadvantageous for the introduction of the resins to the mixer to beseparate from that of the fibers and/or pellets.

Care has to be taken here that the temperature of the homogeneousmixture does not exceed the temperature of about 120° C. at which fullcuring of the resin, or of the resins, if a mixture of two or morehot-curing resins is used, takes place. Use may be made here ofhot-curing resins 2 with varying extents of condensation, and ofdifferent types. The bulk density of the fibers is from about 50 toabout 300 kg/m³ and in particular from about 130 to about 150 kg/m³ andfrom about 250 to about 270 kg/m³. The mixer 1 is composed, for example,of a number of compartments and has an effective length of about 40 D, Dbeing the diameter of the mixer screw. The fibers 3 are homogeneouslyincorporated into the hot-curing resin 2 by the kneading equipment. Theresin is metered volumetrically by a piston pump into the firstcompartment of the mixer. Fibers with a bulk density of about 50 kg/m³,for example, are metered together with another type of fiber whose bulkdensity is from about 250 to about 270 kg/m ³, in particular about 256kg/m³. The fiber type with the low bulk density here is meteredvolumetrically, whereas the fiber type with higher bulk density ismetered gravimetrically. These materials are metered into the third andfifth compartments of the mixer, for example. It is also possible formore than two different types of fiber to be fed, differing from oneanother in their densities and ratio of fiber length to fiber diameter.

In another procedure, just one single type of fiber with bulk densityfrom about 250 to about 270 kg/m³, in particular about 256 kg/m³, isintroduced into the third and fifth compartment. In another instance, asingle type of fiber with a bulk density of from about 130 to about 150kg/m³, in particular about 140 kg/m³, is fed into the resin via thethird and fifth compartment by gravimetric metering.

Besides fibers, use may also be made of pellets made from cellulose orwood. The pellets are produced by compressing fibers made from wood orcellulose, without using any binders. Pellets may also be obtained fromwood flour, fibers of A or B type, or from other types of fiber, bycompression. The bulk density of the pellets is between about 400 andabout 500 kg/m³ and is inversely correlated with the dimensions of thepellets. Use may be made here of combinations made from different fibertypes, fibers and pellets, or made solely from types of pellet. Theselection of any particular combination depends on the density desiredfor the preform to be produced.

The table below gives data for some mixing specifications for preforms.The preforms generally have a moisture content below about 10% by weightand extending to about 2 to about 5% by weight.

Resin Pre- Bulk type Through- Temper- form Wood fiber type density (% byput ature No. (% by weight) (kg/m³) weight) (kg/h) (° C.) 1 A (70) 256Phenolic 100 60 resin (30) 2 A (70) 256 Phenolic 100 60 resin (30) 3 B(70) 140 Phenolic 100 100 resin (30) 4 C (42) + A (28) 50 + 256 Phenolic100 100 resin (30) 5 C (42) + A (28) 50 + 256 Phenolic 100 60 resin (30)6 B (70) 140 Phenolic 100 60 resin (30) 7 B (70) 140 Phenolic 100 120resin (30)

All of the mixes could be processed without difficulty in the mixer, andthe only difficulty experienced was in the metering of a particular typeof wood fiber, namely type B, which tended to bridge. The difficultieswith feeding could be eliminated very simply by using pellets instead offibers. Using these, the throughput given in the table above can beincreased by up to about 100%. The relatively high level of friction inthe mixer breaks down the pellets to give fibers, which become enclosedby the resin(s). Substantially homogeneous incorporation of the woodfibers per se into the resin took place at temperatures of from about 60to about 120° C.

It is useful to knead the mixture until at least substantially uniformor homogenous in a mixer and then pass the same onward to an extruderfrom which the mixture is extruded without any back-pressure beingexerted on the extruder screw. The homogeneous mixture is supplied fromthe mixer 1 to an extruder 4, whose screw configuration has beenappropriately adapted for extruding the mixture made from fibers andresins. The fibers have been enclosed by the resin.

The extruder comprises zones with different temperatures, thetemperature being controlled so that the temperature of the mixture isnot more than about 120° C. at the exit from the extruder. Thecompression zone of the extruder may be cooled in order to ensure thatin no section of the extruder 4 does the mixture exceed the temperatureof about 120° C. above which full curing of the resin begins andproceeds rapidly to completion. If short fibers are used, there is thenso little friction in the compression zone of the extruder that heating,rather than cooling, of this zone is needed. The resin or polymercomposition preferably starts to at least partially cure or polymerizeat this temperature, which temperature however is preferably not highenough for full and entire polymerization or curing of the resin or ofthe mixture made from resin with fibers

The homogeneous mixture made from fibers and resin and discharged froman extruder 4 is introduced into a calender 5 whose nip can be set atfrom about 0.4 to about 10 mm. The homogeneous composition or mixture isspread by the rolls of the calender 5 to give a mat width from about 100to about 4000 mm, and then introduced to finishing equipment 7 viaconveying equipment 6, for example a conveyor belt. In the finishingequipment 7 the length of the preform mat is cut to give the formatsdesired, which are then further processed to sheets, or placed inintermediate storage.

FIG. 2 shows that the mixer can be dispensed with in producing thepreforms, and instead the resin 2 and two different types of fiber 3 aremetered into an extruder 8 which is a twin-screw extruder with kneadingequipment arranged downstream of the inlet aperture for fibers 3. Thekneading equipment is nonreversing elements utilized for achieving amore thorough degree of mixing. Reversible kneading elements cannot beused, since these would cause excessive pressure of the mixture madefrom fibers and resin onto the screw, resulting in blocking of the same.The temperature rise brought about by friction in the extruder can berestricted by using an open-ended extruder, i.e. one which has no die,thus preventing any back-pressure from arising in the extruder. Thismeasure also permits the use of kneading elements to improve thethoroughness of mixing. The temperature of the extruder is thusregulated and controlled so that it does not exceed about 120° C. eitherwithin the extruder or at the exit from the extruder, since attemperatures above about 120° C. full and entire polymerization orcuring of the thermosetting polymer composition(s) or resin(s) begins totake place. The extent to which the material is heated in the extruder 8generally makes it unnecessary to use additional heating, e.g. outsidethe extruder.

As in the procedure in FIG. 1, the extruded mixture is passed to thecalender 5, and from this to finishing equipment 7 via conveyingequipment 6.

FIG. 3 is a diagram of the equipment for carrying out the process inFIG. 2. An extruder 9, corresponding to the extruder 8 in FIG. 2, has upto ten heating zones whose temperatures have been controlled to cover atemperature range of from about 27° C. to 35° C. and, respectively, fromabout 50°C. to 60° C. to from about 106° C. to 115° C. and not more thanabout 120° C. at the exit from the extruder. A resin 2 or a resinmixture made from two or more curable resins is first introduced to theextruder followed by, in succession, a variety of fibers 3. The moisturecan be removed from the resins and from the fibers by reduced-pressuredevolatilization in the extruder 9. It is also possible for the moistureto be removed downstream of the kneading elements of the extruder, byusing an extruder barrel which is open at that location, so that thetemperature prevailing, from about 100° C. to about 120° C., can causethe moisture to evaporate. From the extruder 9 the mixture 11 isextruded continuously onto an inclined supply device 10 which supplies acalender, for example ones having three calender rolls 12, 13, 14. Thecalender may also embrace four to six calender rolls. The supply device10 is usually a conveyor belt which has an adjustable reciprocatingmovement across the width of the mixture 11 in the first nip between thetwo calender rolls 12 and 13. Since more mixture is drawn off at theterminal points of the reciprocating movement of the conveyor belt thanin the middle of the calender rolls 12, 13, the conveyor belt pauses fora certain period at the terminal points. The belt speed for the conveyorbelt and the extent and speed of the traverse movement are adjustable,as is the period of pause at the terminal points. Instead of atraversing conveyor belt it is also possible to use any other conveyorequipment which carries the mixture 11 to the calender rolls. The supplydevice 10 in particular permits the processing of relatively longfibers. In the event that the extruder 9 supplies material to a slotdie, a precondition for which is that the fibers being processed arerelatively small, the supply device and in some cases the calender canbe dispensed with, and the mixture 11 introduced directly from the slotdie to the belt conveyors. The pressure exerted by the calender rollsonto the mixture 11 is such as to mold a stable mat in the nip betweencalender rolls 13 and 14. The width of the mat is from about 100 toabout 4000 mm. After leaving the calender, the preform mat passes onto abelt conveyor 15, 16 which stabilizes the preform mat. An example of abelt conveyor is one composed of two endless conveyor belts which runaround rollers and whose action on the preform mat is similar to that ofa continuous press, and which form a gap through which the preformproceeds.

The curing of the resin or of the resin mixture in the preform isterminated by the calender rolls 13, 14, which are heated totemperatures below about 120° C. or cooled to room temperature or below,or by the conveyor belts 15, 16, since each of these cools the preformto below about 120° C. The density of the preform can be controlled forrelatively high densities by regulating the temperature of the mixture11. The density is from about 250 kg/m³to about 1250 kg/m³, and maymoreover be controlled within this range by using shorter fibers forhigher density.

FIG. 4 is a diagram of other equipment for carrying out the process inFIG. 2. An extruder 17 has up to ten heating zones whose temperaturescover a temperature range of from about 27° C. to about 35° C. and,respectively, from about 50° C. to about60° C. to from about 106° C. toabout 115° C. and not more than about 120° C. at the exit from theextruder. A resin 2 or a resin mixture made from two or more curableresins is first introduced to the extruder 17 followed by, insuccession, a variety of fibers 3 or pellets or a variety of fibers andpellets. The mixture is continuously extruded onto an inclined supplydevice 18 which supplies a conveyor belt 19. This supply device 10 inFIG. 3 and its function have been described above in connection withFIG. 3, to which reference is made here in order to avoid repetition.The supply device 18 spreads a mat on the conveyor belt 19. Two rollers20, 21 compact this mat to give a stable mat 24 of width from about 100to about 4000 mm. As required, the rollers 20, 21 are either cooled, sothat the hot spread mat is cooled, or are heated if the mixture from theextruder has to be raised to the temperature required for partialcuring. The stable mat 24 passes through a gap between two conveyorbelts of a belt conveyor 22, 23, whose action on the mat 24 is similarto that of a continuous press, and which stabilize the mat to give afinished preform.

Advantages achieved by the invention are that preforms of differentwidths can be produced and that fibers of different length and width areused, and that use can be made of resins differing in their extent ofcondensation or in the way in which the resins have been mixed with oneanother or in the types of resin used, and that these are materials notusually suitable for applying resin on a blowing line. In the mixturesmade from fibers and resin, two or more resins can be mixed with thefibers, and separate introduction of these resins to the mixer and,respectively, the extruder is possible here. This makes it possible touse a resin mixture adapted to achieve the desired quality and nature ofthe surface of the preform. If no decorative layers and no coverings areused in producing the sheets from the preforms, these may be givengreater dimensional stability or a particular desired appearance byadding additives, such as mineral particles, radiation stabilizers,catalysts, or color pigments, to the mixture made from fibers andhot-curing resin and, respectively, from curable resins. The extent ofaddition of the additives is from about 1 to about 20% by weight, basedon the weight of the preform. The preforms are used as starting materialfor producing hot-pressed, fully cured sections or sheets, the completeand full curing of the preforms, which have not been entirely cured,taking place during hot-pressing. The preforms here may be hot-pressedas a single layer or, depending on the desired thickness of the sheet,two or more layers of preforms may be hot-pressed together withdecorative layers or with other layers. During the hot-pressing it isalways a requirement that the resin or resin mixture of the preformbecomes completely and fully cured. The resin(s) of the preformshas/have a viscosity of at least about 0.05 Pa.s, and up to about 100Pa.s. Different types of fiber give marked differences in the density ofthe preforms. The higher the bulk density of the fibers, the higher thedensity of the preform. One of the findings here is that when using woodfiber type A, as in the table, with a bulk density of about 260 kg/m³the density of the preform is fairly independent of the temperature inthe range from about 60° C. to about 100° C. In contrast, the density ofthe preform which uses wood fiber type B with a bulk density of about140 kg/m³ is temperature-dependent in the temperature range from about60° C. to about 120° C., the density increasing as temperature rises,giving a density of about 450 kg/m³ at about 60° C. and a density ofabout 550 kg/m³ at about 120° C. Using this type of wood fiber, preformdensities of from 500 to 700 kg/m³ can be achieved in the temperaturerange from about 60° C. to about 120° C. by increasing the pressure inthe calender and increasing the temperature. Wood fiber type A, which issubstantially cubic in shape, cannot be further compacted to anysignificant extent by increasing the temperature, and has an almostconstant density of between about 590 kg/m³ and about 660 kg/m³ in thetemperature range from about 60° C. to about 120° C., even if thepressure is increased. Using wood fiber type B in particular, preformswith a very smooth surface can be produced. At wood fiber bulk densitiesbelow about 140 kg/m³, the preforms obtained have a slightly roughenedsurface structure.

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be readily appreciatedby those of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the claims be interpreted to coverthe disclosed embodiment, those alternatives which have been discussedabove and all equivalents thereto.

1. A preform for sheets, made from a mixture composed of fibers and/orpellets made from cellulose or from wood and at least one hot-curingresin, produced by a process comprising the steps of (a) mixing fromabout 50 to about 90% by weight of fibers and/or pellets with from about10 to about 50% by weight of hot-curing resin, where each of thepercentages by weight is based on the total weight of the mixture, (b)kneading and homogenizing the mixture made from fibers and/or pelletswhich break down to nave fibers, and from at least one hot-curing resin,and passing the mixture onward to an extruder, (c) heating the mixturein the extruder to an extruder exit temperature at which the resinbegins to cure, (d) extruding the mixture, which has not entirely cured,as a homogeneous composition and distributing the homogeneouscomposition within a calender line, (e) exerting pressure onto thehomogeneous composition in the calender line, to an extent which gives amat which is a preform, and (f) removing the preform and stabilizing itsdesired shape.
 2. The preform as claimed in claim 1, wherein the fibersare wood fibers whose moisture level is in the range from about 2 toabout 50% by weight.
 3. The preform as claimed in claim 1, wherein theresins and the fibers and/or pellets are predried to a moisture level offrom about 2 to about 5% by weight prior to extrusion.
 4. The perform asclaimed in claim 1, wherein use is made of hot-curing resins withdifferent extents condensation, or of different types, or of mixtures oftwo or more of these resins.
 5. The preform as claimed in claim 1,wherein fibers of one, two or more types, with different densities anddifferent ratios of fiber lengths to fiber diameters, and/or pellets,are fed to the mixer.
 6. The preform as claimed in claim 1, wherein thehot-curing resins have been selected from the group consisting ofphenolic resins, melamine resins, arid epoxy resins.
 7. The preform asclaimed in claim 6, wherein the hot-curing resins have a viscosity of atleast about 0.05 Pa.s.
 8. The preform as claimed in claim 7, wherein theresins have a viscosity in the range from about 0.05 to about 100 Pa.s.9. The preform as claimed in claim 7, wherein the resins are free fromwater and/or solvent.
 10. The preform as claimed in claim 7, wherein theresins comprise water or a solvent and these are removed during theextrusion or after the calendering.
 11. The preform as claimed in claim1, wherein additives selected from the group consisting of mineralparticles, radiation stabilizers, catalysts and color pigments are addedto the mixture made from fibers and/or pellets and from hot-curingresin(s).
 12. The preform as claimed in claim 11, wherein use is made offibers with a bulk density of from about 50 to about 300 kg/m³ and ofpellets with a bulk density of from about 400 to about 500 kg/m³. 13.The preform as claimed in claim 1, wherein the temperatures andpressures applied to the mixture made from fibers and from hot-curingresin(s) and to the preform mat are selected so as to give the finishedperform a density of from about 250 to about 1250 kg/m³.
 14. The preformas claimed in claim 1, wherein the width of the preform mat is in therange from about 100 to about 4000 mm.
 15. A preform for hot-pressed,fully cured sheet sections or sheets made from a mixture made fromfibers and from at least one hot-curing resin, wherein the mixture iscomposed of from about 50 to about 90% by weight of fibers and fromabout 10 to about 50% by weight of at least one hot-curing resin; wherethe percentages by weight are based on the total weight of the mixture,and wherein the resin of the preform has not entirely cured.
 16. Thepreform as claimed in claim 15, wherein the fibers are wood fibers witha bulk density of from about 50 to about 300 kg/m³.
 17. The preform asclaimed in claim 15, whose density is from about 250 to about 1250kg/m³.
 18. The preform as claimed in claim 15, wherein the hot-curingresin is a resin selected from the group consisting of phenolic resins,melamine resins, and epoxy resins, and wherein the resin is a singleresin or a mixture of two or more resins selected from the group. 19.The preform as claimed in claim 15, which comprises additives, such asmineral particles, radiation stabilizers, catalysts and/or colorpigments at from about 1 to about 20% by weight.
 20. A preform forsheets formed from a mixture comprising cellulose or wood fibers and/orpellets and from at least one thermosetting polymer composition, whereinthe mixture comprises from about 50 to about 90% cellulose or woodfibers and/or pellets by total weight of the mixture and from about 10to about 50% of at least one thermosetting polymer composition by totalweight of the mixture, and wherein the thermosetting polymer compositionof the preform has at least partially polymerized.